1. Field of the Invention
The present invention generally relates to systems for control of batch processes and particularly to automatic systems for providing sequential control and fault detection of discrete devices in batch processes.
2. Description of the Prior Art
A batch process is a process that requires the performance of several sequential steps to arrive at a specific result. Once the result has been achieved the batch process stops until the process is again activated from the first step in the sequence.
Several industrial products are manufactured through the use of batch processes. Batch processes play major roles in the pharmaceutical, pulp and paper, and food processing industries.
Each individual step that is part of an industrial batch process requires the operation of several different discrete devices such as motors and on/off valves. These discrete devices then can be broken down into simple apparatus, such as a motor starter contact and the motor itself, or a valve limit switch used by an on/off valve.
Presently, these batch processes have been controlled with distributed process control computer systems such as the Bailey Controls Co. NETWORK 90 system. Such process control systems are made up of control elements that may make up several different control schemes. The control elements are made up of microprocessors which contain several algorithms in their ROM's (read only memories). Each algorith is used to perform a logic or control function and is called a function block. Some examples of function blocks include simple logic blocks, such as OR and AND blocks, and complex control blocks, such as a PID (proportional-integral-derivative) controller block. The numerous microprocessor control elements of the computer energize different function blocks to provide numerous control configurations as required.
A present apparatus using a process control computer system for batch control includes the control of discrete devices through conventional function block logic. Controlling the discrete devices through conventional function block logic requires a one to one match between a function block control scheme and each simple apparatus which makes up the discrete devices. The reason for this is that each simple apparatus requires a feedback logic scheme to signal when it has completed its operation. This feedback signal is used to determine if the simple apparatus has malfunctioned and to energize other discrete devices in that particular step of the batch process.
Controlling one discrete device and signaling the operator when a simple apparatus of the device has malfunctioned often requires ten or more function blocks and becomes rather cumbersome. This requires the use of much microprocessor power and is a waste of computer memory space. Also, the time to process the information from the discrete device becomes prolonged due to all the functions required. If the batch process requires the use of several discrete devices, which is often the case, the cost of microprocessor power becomes a big problem.
Another problem associated with using conventional logic to control discrete devices in batch control is that it requires two manual interface buttons. One button is required to set the discrete device in manual or automatic mode and the other is to turn the discrete device on or off. This brings the element of operator error into the system. A batch process uses several discrete devices at one time and having two manual control buttons for each device is difficult for the operator when trying to control the entire process. Also two messages, one for each button, must be sent to the rest of the control system instead of just one for a singular discrete device. This creates a tremendous amount of unnecessary loading upon the distributed computer control system.
Thus it is seen that an automatic system for the control and fault detection of discrete devices used by a batch process is needed which eliminates extensive function block configurations and system loading in distributed process control computer systems.